Conventionally, paper containers are frequently employed as beverage cups, food trays, etc., and most of these containers are provided by a molding method using bonding or thermal welding. When an adhesive or thermal welding is employed, deep containers (deep cups) can be produced. Further, a drawing process for thermoplastic resin laminated paper, or a bulk molding compound forming process using a paper making press apparatus, for emulsifying pulp fiber with a solution, is employed; however, either method is not appropriate for a process for changing a container form, and the obtained product is a thick container, such as an egg container, that has a low commercial value in quality appearance.
A method for manufacturing a deep container by folding a single sheet of blank without employing an adhesive is introduced in patent literature 1, patent literature 2 and patent literature 3. However, patent literature 1 is the invention that discloses paper tableware with a raised bottom, and this tableware is efficient as a disposable tableware used in a campsite, etc., but is inferior in fasionability, and is inappropriate for re-heating by a microwave oven because of the shape of the raised bottom.
Patent literature 2 discloses a deep paper container, as shown in FIGS. 8 and 9, that is produced by integrally molding a single sheet of blank B, and that includes a polygonal bottom face D, an opening top and a peripheral wall face A, which is formed by alternately arranging spiral inner folded faces 4 and partition faces 3 circumferentially along the bottom face, and for which the individual partition faces are joined together along the side edges, and are raised circumferentially with a predetermined inclination, from the peripheral edge of the bottom face to the top opening edge, and the individual inner folded faces are folded in half, into a triangular shape where the vertex contacts the peripheral edge of the bottom face, and overlap the inner surfaces of the corresponding partition faces, and also discloses the manufacturing method for the container. Patent literature 3 is substantially the same.
As shown in FIG. 9, according to patent literature 2, a blank is disclosed, wherein a polygonal portion 12 corresponding to a bottom face is located in the center, and quadrilateral portions 13 corresponding to partition faces of a peripheral wall and triangular portions 14 corresponding to inner folded faces are alternately arranged in a radial manner; the partition face equivalent portions 13 are folded into two in a triangular shape, while small triangles and small quadrilaterals are not defined with ruled lines in the partition face equivalent portions, and therefore, a pattern consisting of raised portions is not provided on the peripheral wall of the container, as shown in FIG. 8. Since the containers disclosed in these patent literatures do not define space with respect to the table of a microwave oven, unlike the container disclosed in patent literature, it is assumed that microwave cooking for the contents can be effectively performed.
However, for these containers, the thickness of paper used for container formation affects the external appearance and the function of a container, and in a case wherein thin paper is employed as a blank, a folding process for obtaining a beautiful external appearance is enabled, while there is still a problem on the function serving as a container, and in a case wherein thick paper is employed, a resistance to bending is high and distortion, such as deformation of an unexpected portion, occurs, and therefore, it is difficult to maintain a desired shape.
When the inventor of the present invention employed the paper container manufacturing method disclosed in patent literature 2, and attempted to fold a sheet of paper pulp having a thickness of 150 microns, the inventor found that it was not easy to make a crease along mountain fold lines and valley fold lines, and the container tended to be open outward. For the reason, the thickness of the product was increased by four or five times of the blank at the individual corner points of the polygonal bottom face where mountain folds and valley folds overlapped, and the folded shape could not be maintained unless considerable compressive stress and considerable pressure for smoothing out the portion were applied.
The paper pulp is an aggregate where natural macromolecules consisting of cellulose are three-dimensionally intertwined. Therefore, when a bending stress, a compressive stress and a tensile stress are applied to this structure, the folded form can not be easily maintained due to the factors, such as a strain reaction of the three-dimensional structure and a long period required for relaxation of the polymer chain from the deforming stress, and the structure tends to be returned to the original shape. In paperfolding, a tensile stress is exerted to the mountain fold, while a compressive stress is exerted to the valley fold. Further, these stresses are proportional to the third power of the thickness of the paper. Only a small thickness of paper is critical for the difficulty of paperfolding. For example, a folding process can be appropriately performed for a paper sheet having a thickness of about 10 microns, while eight times as high as the stress is required for paper of 20 microns.
In the processing for folding practically thick paper pulp and meeting the adjacent inner folded faces to form a deep container, distortion, such as unexpected deformation or so-called collapse, is observed near the bottom part. Therefore, after the deep container is formed, a post-process for pressing the bent portion, for example, is required as a countermeasure in order to correct the distortion. More specifically, in a case wherein a blank disclosed in patent literature 2 or 3 is to be folded along the mountain fold and valley fold lines to form a container, a curved sheet surface (HP surface) that is smooth in the out-of-plane direction is required for the quadrilateral, partition face equivalent portions. Therefore, due to an excessive twist angle formed by the two opposite short sides, an irregular rough distorted surface tends to occur along the two opposite long sides, and especially, convex distortion at the portion near the bottom face equivalent portion is most remarkable. As a result, since it is difficult to form the original, smooth HP surface, various form correction means must be employed after the container has been formed by folding the blank.
It should be noted that, for a quadrilateral flat sheet that can be curved and deformed in the out-of-plane direction, the HP (Hyperbolic Parabolid) surface is a smooth, curved sheet surface that is obtained by rotating the two opposite sides each other in the out-of-plane direction, while these two sides are maintained substantially linear, and that can be returned and restored to the quadrilateral flat sheet by rotation. Furthermore, in the description in patent literature 2 or 3, automatic formation can be performed by employing a cavity with a protrusion and a punch with a groove; however, the embodiments and the manufacturing methods are not provided on the assumption of the performance of a mechanical press formation, and it is reasonable to believe that manual folding is performed.
Paper containers, such as paper cups, are produced by the formation method for adhering the bottom part and the wall part (barrel part), while deep containers are produced by folding a single blank, and therefore, a large amount of materials, such as paper, is required and the formation speed is low, so that the commercial value of a container produced for the same purpose is low, and even in a case wherein a good appearance of the HP surface formed on the peripheral face is obtained by various form correction means, the competitive power on the market is unfortunately still low. Therefore, a further development of the shape and form of a container produced by folding a single sheet of blank has been requested for preventing unexpected deformation and distortion during the folding process, and for utilizing in many fields the essential, beautiful appearance and the function of the HP surface.